15 digits can make for 1,000,000,000,000,000 different card numbers, leaving 16th digit for checksum. This is a completely made-up problem, caused by chopping t

15 digits can make for 1,000,000,000,000,000 different card numbers, leaving 16th digit for checksum. This is a completely made-up problem, caused by chopping the number into card type, bank id, etc.